Mechanical gearboxes are extensively used to match the operating speed of prime-movers to the requirements of their loads for both increasing the rotational speed such as, for example, in wind-powered generators or reducing rotational speed such as, for example, in an electric-ship propulsion arrangement. It is usually more cost and weight effective to employ a high-speed electrical machine in conjunction with a mechanical gearbox to achieve requisite speed and torque characteristics. However, while such a high-speed electrical machine in conjunction with a mechanical gearbox allows high system torque densities to be realised, such mechanical gearboxes usually require lubrication and cooling. Furthermore, reliability can also be a significant issue. Consequently, direct drive electrical machines are employed in applications where a mechanical gearbox cannot be used. Some direct drive electrical machines, such as permanent magnet rotary/linear homopolar (transverse-field) machines (TFM) have poor power factors which make them unsuitable for electrical power generation and require higher converter volt-ampere ratings for motor applications.
Recently, pseudo-direct drive electrical machines have been proposed which provides at least one stator and two moveable elements, such as inner and outer rotors, which interact in a magnetically geared manner via asynchronous harmonics of first and second pluralities of permanent magnets. Such assemblies are described in various embodiments in GB 2 437 568 and U.S. 2004/0108781 which are incorporated herein by reference.
However, such apparatus are required to be designed to be capable of producing a peak torque, dependent upon their intended application, even when typical torque levels to be produced may be significantly lower. Such designed-to-accommodate-peak-torque apparatus are physically large, and consequently expensive to manufacture. Further, such designed-for-peak-torque apparatus have relatively high iron losses formed by a combination of eddy current and hysteresis losses. Finally, the emf that is generated in the windings which are attached to the stator is a function of the speed of the rotors, and cannot be changed to, for example, provide reactive power to the terminals of the electrical machine. It is therefore not possible to directly connect the prior art electrical machine as a generator to the electrical supply grid. It is an object of embodiments of the present invention to at least mitigate one or more of the above prior art problems.